1. Field
This invention is related to winding technology of electric machines, and particularly hairpin wound electric machines.
2. Background Discussion
Electric machines are key components of conventional automobiles. Some electric machines serve as starting motors to crank automobile engines. Other electric machines serve as alternators that generate electricity from engine motion and deliver power to automobile loads. Electric machines are also very important in modern hybrid electric vehicles (HEVs) that combine an internal combustion engine with an electric drive system powered by a battery bank. In these hybrid vehicles, a single electric machine is typically required to operate as (a) a starter motor, (b) an electric drive assist (propulsion boost) as well as pure electric drive (propulsion), (c) a generator providing electric power for onboard electric loads and charging the battery banks, and (d) a re-generator acting to convert the kinetic energy of the vehicle to electric power for charging the battery bank during braking/deceleration of the vehicle.
Hybrid Electric Vehicles can operate with low fuel consumption and low air-pollution. There are two propulsion systems onboard the HEV: (i) the traditional diesel/gasoline engine and (ii) the electric drive system. The additional electric drive system consists of battery bank (energy storage component), power electronics unit (control components) and electric machine (conversion component—electrical to mechanical energy). The electric drive system provides propulsion and power generation as well as power regeneration.
The electric machine is a core component in the HEV's electric drive system. Based on driving schedule/requirements, the machine will run under the motoring state during vehicle starting, electric assist drive/propulsion or pure electric drive/propulsion. The electric machine is required to operate under the normal generating state during engine drive/propulsion (thereby charging batteries) and the re-generating state during vehicle braking. Obviously the efficiency of the electric machine will directly govern the efficiency of electric drive system and consequently the fuel economy of the vehicle.
As the power conversion component in an electric drive system, the electric machine interacts directly or indirectly (through a belt or mechanical converter/clutch) with the drive shaft or engine shaft and has to be located ‘under the hood’ of the vehicle. The space available for the electric machine in the required location is limited. Therefore, the size/dimensions of the electric machine must be as small as possible. Compared to other applications for electric machines, high efficiency and small size are more important to the electric machine onboard the HEV. However, automobile manufacturers are increasingly calling for high efficiency and small size for almost all electrical machine applications in vehicles. Therefore, the need for small and mid-sized electric machines having high efficiency and small size is applicable to all automotive vehicles, and is particularly applicable to HEVs and purely electric vehicles.
One way to reduce the size of an electric machine and increase efficiency is to increase the slot-fill-ratio of the electric machine. With reference to FIGS. 1(a) and 1(b), small and mid-sized electric machines include a stator 26 formed from a lamination stack. A plurality of slots 20 are formed in the stator. The stator slots are arranged in a circular fashion around the stator with an opening 22 to the slot that faces the rotor of the electric machine. The slots 20 of these electric machines are deemed “partially closed” or “semi-closed” because a neck 28 is formed near the opening to each slot, such that the width of the opening 22 is smaller than the width of the slot itself. A plurality of electric conductors 24, typically in the form of copper wires, are positioned in the slots of the stator.
As mentioned in the preceding paragraph, to design an electric machine with high efficiency and small volume, a high slot-fill-ratio (SFR) is preferred. The term “slot-fill-ratio” is typically defined as the ratio of (a) the aggregate cross-sectional area of the bare copper conductors in a slot to (b) the cross-sectional area of the slot itself. With high SFR, the large cross-sectional area of the copper wires helps reduce the phase resistance and consequently the resistance of the windings (i.e., power loss) for a given slot size, so the efficiency of the machine is improved. Accordingly, more efficient electric machines can be built at a smaller size than less efficient predecessors.
Armature windings of most small and mid-sized electric machines are typically wound in many turns with single or multiple strands of round magnetic wire. FIG. 1(b) shows an exemplary prior art stator slot having a plurality of round conductors in the stator slot. The SFR of the round wire machines can reach a maximum of 44% preventing the design of low loss (resistance), high efficiency electric machines. As discussed previously, this issue becomes even more critical when designing high efficiency machines for hybrid vehicles. Available space onboard hybrid vehicles is strictly limited, and therefore, boosting efficiency by increasing machine size becomes impractical.
One solution to increasing the SFR is to use rectangular wires in the stator slots in place of round wires, such as the arrangement shown in FIG. 1(a). Use of rectangular wires in the stator slots can increase the slot-fill-ratio up to 70% over that of round wires, allowing the SFR of rectangular wire machines to reach near 75% or higher. Unfortunately, the phenomena known as “skin effect” limits the size of conductors that may be used in the stator slots, especially the thickness of the conductor in the slots. “Skin effect” reduces the effective cross-sectional area of a conductor in a slot as the thickness of the conductor increases. Skin effect is especially prevalent in straight conductor segments at high speed operation. Accordingly, use of rectangular wires can increase the SFR, but the thickness of each rectangular wire relative to the slot height/depth has to be limited in order to reduce the skin effect of the conductors. Because the leakage flux linkage at different height/depth levels of a conductor in a given slot increases from the top to the bottom of the slot, the back EMF corresponding to the lower part of the conductor becomes higher than that at the top part of the conductor.
The back EMF due to slot leakage flux forces a great amount of current flow at the top part of the conductor. Accordingly, the current density in the conductor in a slot increases from the bottom of the slot to the top if all the conductors in the slot carry the same phase current as set forth in equation (1) below. If different phase currents are carried in a slot, the current density exhibits a complicated distribution. When the AC current flows in a conductor, skin effect will reduce the effective cross-sectional area of the conductor so the AC resistance of a conductor is larger than its DC resistance. The increase of AC resistance due to skin effect in rectangular slot depends on the penetrated depth, d, of electromagnetic wave, i.e.,                     d        =                                                            b                s                                            b                c                                      ⁢                          ρ                              π                ⁢                                                                   ⁢                f                ⁢                                                                   ⁢                μ                                                                        [                  equation          ⁢                                           ⁢                      (            1            )                          ]            Where ρ is the resistivity of the conductor; f is the frequency of the AC signal; μ is the permeability of the conductor (roughly equal to the permeability of air for copper conductors), bc and bs are the widths of the conductor and the slot, respectively. Obviously the penetrated depth of electromagnetic waves will be reduced as frequency increases. Generally, there will be no current flowing at the lower part of the conductor in a slot if the thickness of the wire is triple the penetrated depth. Therefore, the thickness of rectangular wires in the slot should be made as small as possible.
If several strands of wires are laid in a slot, the leakage back EMF in a strand in the lower part of the slot will be higher than the leakage back EMF in the strand in the upper part of the slot under the effect of slot leaking flux. Circulating currents among the strands will be produced due to the unequal leakage back EMFs if the strands are welded together as one conductor at the end-turn segment, which will raise the power loss. To reduce or eliminate the circulating current in large electric machines, the winding bars are often composed of many strands of small/thin rectangular conductors whose positions are transitioned (such as 540° transitioned winding bar etc.) in the axial direction of a slot (see FIG. 2). Reducing the wire thickness in the stator slots helps to lower the negative skin effect and assists in achieving high efficiency in an electric machine with rectangular conductors. However, these winding bars have to be laid in open slots requiring special slot wedges (normally a magnetic slot wedge to reduce skin and air-gap effects). Because of this, such winding bars are too complicated to utilize in manufacturing of small and mid-sized electric machines, as they would dramatically increase the difficulty and cost of manufacturing, and reduce the reliability of the machines, especially in hybrid vehicle applications. Furthermore, it is not desirable to use open slots in many small and mid-sized electric machines. In middle and small size high frequency AC machines, Litz wires are often used for AC windings. Litz wires can help reduce circulating current and skin effects of AC windings, but the slot-fill-ratio cannot be improved with Litz wires. On the other hand, the transition of AC windings should be performed by a special design according to armature stack length in order to eliminate circulating current within windings, simply picking up available Litz wires may not reach the goal of eliminating circulating current. Furthermore, Litz wire windings are very difficult to handle during manufacturing, and would therefore contribute to manufacturing costs and present additional manufacturing hurdles.
To simplify the manufacturing and the keep high slot-fill-ratio of windings, pre-formed rectangular wires have been formed having straight conductor segments that are positioned in the stator slots, but twisted ends that form the end turns, as shown in FIG. 3. However, the pre-formed windings shown in FIG. 3(a) must be inserted through the slot opening to be inserted on the stator. Thus, these pre-formed windings can only be placed in open slots like the AC windings of FIG. 2 that are used for large electric machines. These windings can not be placed in the partially closed slots typically used for small and mid-sized electric machines because the restricted opening in a partially closed slot prohibits the windings from entering the slot.
To solve this problem, designers of small and mid-sized electric machines having partially closed slots have used conductor segments that may be inserted into the top and/or bottom of the slot and need not pass through the slot opening. To this end, the conductor segments are first bent into U-shapes, such that the conductor segments form a U-shaped end turn with two legs, such as that shown in FIGS. 7(a) and/or 7(b). These conductor segments are often referred to as “hairpins” because of their shape. The U-shaped conductor segments may be inserted into the slots from one side of the lamination stack, legs first, with each leg positioned in a different slot. The leg ends of the hairpins extending through the slots (i.e., the open ends of the hairpins) are then bent to a desired configuration, as shown in FIG. 7(c), so each respective leg end may be joined to a different leg end according to the connection requirements of the windings. Finally the corresponding rectangular wires are connected into 3-phase or multi-phase AC windings.
This “hairpin” winding technology is already in application in many products such as Delco Remy America, Inc.'s 50DN alternator (since the 1960's), shown in FIG. 3(b). The configuration of the windings shown in FIG. 3(b) (i.e., double layer windings with one strand per conductor) leads to high conductor thickness, which (among other reasons) causes severe skin effect and low efficiency (less than 50%).
To lower the skin effect while maintaining the high slot-fill-ratio of rectangular wire AC windings, each conductor could be composed of a plurality of thin rectangular wires or “strands”, as shown in FIG. 4(a). Like the above-described hairpin winding process, the conductor with multi-strands could be transitioned at the end-turn, shaped into U-shapes and inserted into the slots from one side of the lamination stack. After all U-shape coils are inserted into slots, the open segments of the U-coils would be reshaped to the required shapes and connected into phase windings. One prototype of this technology is shown in FIG. 4(b). However, because of limitations of the number of slots and the available end-turn space, the practical application of FIGS. 4(a) and 4(b) is wave windings with one turn per coil since connecting wires between poles at lap windings could spoil the end-turn space. Therefore, this technology can be only used for low voltage AC windings like the FIG. 4(b) prototype. Furthermore, another manufacturing issue exists because of the difficulty in twisting multi-strand rectangular wires.
Another solution to the above-referenced problems involves shaping each strand into a single turn coil and then two U-shape coils are inserted into the slots, as shown in FIG. 5(a), instead of combining two strands in one conductor as shown in FIG. 4(a). Following the same procedure as hairpin winding manufacturing in FIG. 3(b), an automotive alternator which uses two sets of overlapping windings (i.e., one on top of the other) being connected with two separate rectifiers in parallel can be created. However, besides requiring two bridges, the overlapped windings increase the difficulty in manufacturing and the possibility for short-circuits. Furthermore, added end-turn length is required for overlapped windings (i.e., the size of the winding head is increased). This added length is undesirable in modern vehicles, such as the HEV where machine space is of much concern. In addition, the repair of overlapped windings is very difficult if not impossible, as one set of windings completely encompasses the other set of windings.